1. Field of the Invention
The present invention relates to a method for pattern recognition. More particularly, the present invention relates to a method for detecting the orientation of a normal lead frame or an inverted lead frame during a wire bonding process and also to detecting the orientation of symmetrical dies (semiconductor chips).
2. Description of the Related Art
In general, wire bonding is a process that connects leads to each other. For example, in die wire bonding, bond pads formed on the surface of the die are electrically connected to leads of a substrate such as a printed circuit board, a circuit tape, a circuit film, a lead frame, or the like. For simplicity, the substrate shall hereinafter be referred to as a lead frame although it is understood that other substrates can be used.
A wire bonding device 900 for wire bonding is shown in FIG. 9. Wire bonding device 900 includes a camera 902 for capturing a picture, a monitor 904 for displaying the picture captured by the camera 902, a memory 906 for storing the positions of a lead frame 908 and a die (not shown), e.g., the positions are input by an initial operator, a central processing unit 910 for performing a general control such as data processing and input/output, a transfer unit 912 for transferring the lead frame 908, and a wire bond control unit 914 for moving and controlling a bond head 916, on which a capillary (not shown) and the camera 902 are mounted, in the axis of X, Y and Z. The above construction is well known.
A conventional method, which recognizes a pattern of the lead frame 908 and of the die using the wire bonding device 900 will be described as follows. First, the construction of a lead frame and a clamp used during the bonding of the lead frame will be described hereinafter.
Referring to FIG. 10A, a typical normal lead frame NLF includes a space 3 of a prescribed size formed at the center and a frame body 2 of a board type formed at both sides of the space 3 for maintaining and supporting the whole structure. A die pad 4 of a rectangular board type to mount a die 30 during the manufacturing process is disposed at the center of the space 3. The die pad 4 has four edges, to which ends of four tie bars 5 are connected respectively. Three of the tie bars 5 are connected to buffing connection boards 18 at the other ends respectively. The buffing connection boards 18 are connected to the frame body 2. The other of the tie bars 5 is connected to a gate 16 (shown at the upper and left edge of FIG. 10A) serving to make resin easily flow toward the die pad 4 during the manufacturing process (molding step). The gate 16 is connected to the frame body 2.
Here, the gate 16 has a dent part 16A formed at one side thereof to indicate the position of the gate 16 and to allow the normal lead frame NLF to be easily discriminated from an inverted lead frame (ILF), which will be described hereinafter. Moreover, each of the tie bars 5 is bent downwardly with a prescribed slant in a specific area in such a manner that the die pad 4, which is connected and supported by the tie bars 5 is located at a lower area than the frame body 2, i.e., is downset.
Meanwhile, a plurality of inner leads 6 are arranged around the die pad 4 radially in prescribed intervals from the die pad 4. Furthermore, the inner leads 6 are connected to a plurality of outer leads 10, respectively. Ends of the outer leads 10 are integrally connected to straight support bars 12. Additionally, the straight support bars 12 are connected to a plurality of bent support bars 14. The bent support bars 14 are connected to the frame body 2.
Between the inner leads 6 and the outer leads 10, a dambar 8 is provided at right angles to the longitudinal direction of the inner leads 6 or the outer leads 10. The dambar 8 serves to prevent resin from overflowing to the outer leads 10 during molding.
FIG. 10B is a plan view showing an example of a conventional inverted lead frame ILF. Note that the gate 16 for injecting resin (shown at the upper and right edge of FIG. 10B) is at a different location compared to the gate 16 of the normal lead frame NLF.
Illustrated in FIGS. 10A and 10B are an adhesive tape 22 for preventing the short or bending of the inner leads 6, an index hole 1 for detecting or fixing the orientation of the lead frame NLF or ILF, and a passivation layer 33, sometimes called a glass or glassification layer, coated on the surface of the die 30.
FIG. 11 is a perspective view of a conventional clamp 1100. use of clamp 1100 with regards to normal lead frame NLF (FIG. 10A) is hereinafter described although it is understood that clamp 1100 is used with inverted lead frame ILF (FIG. 10B) in a similar manner. Referring now to FIGS. 10A and 11 together, the clamp 1100 serves to fix the normal lead frame NLF not to move during the wire bonding process in the state that the normal lead frame NLF on which the die 30 is mounted, is seated on a heater block (not shown) of the wire bonding device 900 (FIG. 9) by the transfer unit 912. The clamp 1100 has a window 1140 being in the form of a quadrangle in such a manner that a prescribed area of the die pad 4, the inner leads 6 and the tie bars 5 of the normal lead frame NLF is opened outward.
Continuously, referring now to FIGS. 12, 13A, 13B, 13C, 13D, 13E and 14, the conventional method for recognizing a pattern for wire bonding will be described.
Referring now to FIGS. 12 and 13A together, in a lead frame orientation detecting step 1202, a sensor senses index holes 1 of the normal lead frame NLF, which is loaded on the heater block (not shown) by the transfer unit 912 (FIG. 9), and it is determined whether or not the normal lead frame NLF is loaded in an exact direction. Here, if the normal lead frame NLF is loaded in the contrary direction, the position and the number of the index holes 1 are changed, and thereby the bad loaded state of the normal lead frame NLF can be sensed. Furthermore, at this time, the normal lead frame NLF is not completely clamped by the clamp 1100.
Referring now to FIGS. 12, 13A and 13B together, in a first lead frame indexing step 1204, a camera, e.g., camera 902 of FIG. 9, and a pattern recognition system (PRS), which converts a picture captured by the camera into an electric signal, set a lead eye box LEB1 and a lead eye point LEP1 on one tie bar 5, for example, on the tie bar 5 located at the upper and left end inside the window 1140 of the clamp 1100 of the normal lead frame NLF as best shown in FIG. 13B. Generally, a lead (die) eye box is an image of an area and a lead (die) eye point is a specific location, i.e., point, within the lead (die) eye box.
The camera and PRS capture the picture, and it is determined whether or not the captured picture is identical with a first control picture previously stored in the memory, e.g., in memory 906 of FIG. 9, within the permitted range. If the captured picture is identical with the first control picture stored in the memory, the next step is progressed. If the captured picture is identical with the first control picture within the permitted range, the normal lead frame NLF is moved in the axes of X and Y, e.g., horizontally and vertically in the view of FIG. 13A, to make the captured picture be identical with the first control picture. Moreover, if the captured picture is different from the first control picture beyond the permitted range, further steps are stopped and an operator's input is waited for.
Here, the PRS is the most advanced technique of picture information processing systems. The PRS is widely used for semiconductors, measuring instruments, material analysis, medical science fields and military affairs. Such PRS is applied to the semiconductor field, especially, the wire bonding device 900 (FIG. 9). The general principle of the PRS is that the control picture stored in the memory and the picture captured by the camera are compared and a determination is made whether or not the captured picture is identical with the control picture. If the captured picture is identical with the control picture to within the permitted range, the normal lead frame NLF or the camera is moved in the axes of X and Y to make the captured picture of the lead frame be identical with the control picture stored in the memory.
Referring now to FIGS. 12, 13A, 13B and 13C together, after the normal lead frame NLF is clamped with the clamp 1100, in a second lead frame indexing step 1206, the camera and the PRS set the first lead eye box LEB1 and the first lead eye point LEP1 on one tie bar 5 of the normal lead frame NLF as best shown in FIG. 13B. A determination is made as to whether or not the captured picture is identical with the first control picture stored in the memory within the permitted range. If the captured picture is identical with the first control picture stored in the memory, the next step is progressed. If the captured picture is identical with the first control picture to within the permitted range, the camera is moved in the axes of X and Y to make the captured picture be completely identical with the first control picture.
Continuously, a second lead eye box LEB2 and a second lead eye point LEP2 are set on another tie bar 5, for example, on the tie bar 5 located at the lower and right end inside the window 1140 of the clamp 1100, of the normal lead frame NLF as best shown in FIG. 13C. The picture is captured and a determination is made as to whether or not the captured picture is identical with a second control picture stored in the memory within a permitted range. If the captured picture is identical with the second control picture, the next step is progressed. If the captured picture is identical with the second control picture to within the permitted range, the camera is moved in the axes of X and Y to make the captured picture be completely identical with the second control picture.
Here, when the first and second lead eye boxes LEB1, LEB2 are set, if either of the captured pictures are different from the respective control pictures beyond the permitted range, further steps are stopped and the operator's input is waited for. That is, the operator sets the lead eye boxes LEB1, LEB2 and the lead eye points LEP1, LEP2 manually to set the normal lead frame NLF in an exact position.
In a Video Lead Locator (VLL) step 1208, the camera reads the position of each inner lead 6 of the normal lead frame NLF and stores the position in the memory.
Referring now to FIGS. 12, 13A, 13D and 13E together, in a die orientation detecting step 1210, the edge of the die 30 and two or more bond pads P formed in the vicinity of the edge of the die 30 are set as a first die eye box DEB1 and a first die eye point DEP1 as best shown in FIG. 13D. The picture is captured, and the captured picture is compared with a third control picture stored in the memory. If the captured picture is identical with the third control picture stored in the memory, the next step is progressed. If the captured picture is identical with the third control picture within the permitted range, the camera is moved in the axes of X and Y to make the captured picture be completely identical with the third control picture, and then, the next step is progressed. If the captured picture is different from the third control picture beyond the permitted range, further steps are stopped and the operator's input is waited for (the operator then finds and inputs the first die eye box DEB1 and the first die eye point DEP1 manually).
Continuously, in the same way, a second die eye box DEB2 and a second die eye point DEP2 are set in the vicinity of another edge of the die 30 as best shown in FIG. 13E. After the same steps are performed, if the captured picture is completely identical with a fourth control picture stored in the memory, the next step is progressed. If the captured picture is identical with the fourth control picture within the permitted range, the camera is moved in the axes of X and Y to make the captured picture be completely identical with the fourth control picture, and the next step is progressed. If the captured picture is different from the fourth control picture beyond the permitted range, further steps are stopped and the operator's input is waited for.
Continuously, referring now to FIG. 14, a virtual straight line 35 is drawn between die eye points DEP1, DEP2 and a central point CP of the virtual straight line 35 is compared with a control central point stored in the memory (e.g., stored in the memory by the operator when the die is loaded initially) . If the central point CP is identical with the control central point within the permitted range, the next step is progressed, but if they are not identical, further steps are stopped and the operator's input is waited for.
Continuously, based on the central point CP, the positions and the coordinates of all bond pads P (i.e., bond pads P1, . . . , Pn) of the die 30 are calculated.
For example, if the position and the coordinate of a first bond pad P1 from the central point CP is calculated, the positions of the remaining bond pads P2, P3, . . . , Pn can be all calculated. In more detail, since the relative or absolute positions of all bond pads P are initially stored in the memory, if only the position of the first bond pad P1 is found, coordinates of the remaining bond pads P2, P3, . . . , Pn can be automatically found.
In FIGS. 13D, 13E and 14, probe marks 31 are formed by contacting bond pads P with a probe when the die 30 is tested in the electrical efficiency and a passivation layer 33, which protects the upper surface of the die 30 from the outside is illustrated.
Referring now to FIGS. 12, 13A, 13B, 13C, 13D and 13E, in a wire bonding step 1212, the bond pads P of the die 30 and the inner leads 6 of the normal lead frame NLF are bonded with a conductive wire using the positions and the coordinates of the die 30 and the normal lead frame NLF and the capillary of the bond head. The bonding is started from the first bond pad P1 of the die 30 and a first inner lead 6 of the normal lead frame NLF.
Even though the normal lead frame NLF or the die 30 are tilted or have error in the position, if they are within the permitted range, the bond pads P of the die 30 and the inner leads 6 of the normal lead frame NLF can be all wire-bonded.
However, the conventional pattern recognition method and clamp have the following problems.
First, the lead eye boxes LEB1, LEB2 and the lead eye points LEP1, LEP2 for the first and second lead frame indexing steps 1204, 1206 are set on the tie bars 5 located inside the window 1140 of the clamp 1100. Since the normal lead frame NLF is symmetric, if the normal lead frame NLF is inadvertently rotated at an angle of 180 degrees when it enters the wire bonding, this inadvertent rotation, sometimes called misalignment, cannot be detected. The structure of the die pad 4 and of the tie bars 5 connected to the die pad 4 shown through the window 1140 of the clamp 1100 are in a complete symmetrical form, and thereby, if the normal lead frame NLF is rotated at an angle of 180 degrees, the wire bonding device cannot detect it. Such problem occurs more frequently when the normal lead frame NLF and the inverted lead frame ILF are used together.
Even though, in the lead frame orientation detecting step 1202, the index holes 1 are counted and the direction is detected, the detection is performed inaccurately due to pollution of the normal lead frame NLF or the sensor. Therefore, there is much possibility that the misaligned normal lead frame NLF will pass the lead frame orientation detecting step 1202.
Second, since the lead eye boxes LEB1, LEB2 and the lead eye points LEP1, LEP2 are located at the center of the heater block on which heat is concentrated, the normal lead frame NLF is rapidly oxidized and the color of the normal lead frame NLF becomes similar with that of the heat block, and thereby the pictures cannot be exactly captured. That is, the picture recognition is lowered.
Third, if the die 30 is inadvertently bonded in rotation at angles of 90 degrees, 180 degrees or 270 degrees, this misalignment of the die 30 cannot be detected and the wire bonding is performed on the misaligned die 30.
That is, recently, the die 30, which has bond pads P of symmetrical type in all directions, is widely used, and sometimes, the die 30 is misaligned and bonded in rotation at prescribed angles, for example, 90 degrees, 180 degrees or 270 degrees, in a die bonding step. In that case, the position of the first bond pad P1 is changed, however, it cannot be detected through the above pattern recognition method. Therefore, the central processing unit incorrectly determines that the die 30 is bonded in the correct position and performs the wire bonding. However, actually, as the first bond pad P1 is located in a different area, the wire bonding of all bond pads P and the inner leads 6 goes wrong. Therefore, only during the electrical test performed after the wire bonding is finished is the defect detected thereby sharply lowering the production efficiency.
Fourth, as the bond pads P are finely pitched, the picture recognition rate by the PRS is lowered when the die eye boxes DEB1, DEB2 and the die eye points DEP1, DEP2 are set. That is, the closer the distance between the bond pads P is, the smaller the area of the bond pads P is, but the size of the probe mark 31 formed during the electrical test of the die 30 is not reduced. Therefore, if the picture inside the die eye boxes DEB1, DEB2 is converted into an electric signal, as the color of the probe mark 31 (for example, black color) takes the larger area than the color of the bond pad P (for example, white color), in the probability, all the bond pads P may be converted into electric signal of black color, and thereby the picture recognition rate is lowered. Therefore, there occurs trouble that the operator must detect orientation of the die 30 manually. Here, the probe mark 31 is a black mark formed by the contact of the probe when the efficiency and the validity of the die 30 are tested.
Fifth, the passivation layer 33 is covered on the surface of the die 30 to protect various circuits to the inside of rows in which the bond pads P are formed, and thereby the color of the surface is shown in various colors or rainbow colors. The phenomenon is more deepened by heat provided during a sawing step of a wafer or various manufacturing steps. By the change of color, the camera cannot exactly recognize the picture inside the die eye boxes DEB1, DEB2, and thereby the picture recognition rate is lowered.